1. Field of the Invention
Embodiments of the invention relate to perpendicular magnetic recording media, and more particularly to a surface treated granular magnetic layer exhibiting enhanced magnetic properties and improved corrosion resistance and contact start-stop performance.
2. Description of the Related Art
Perpendicular magnetic recording media are being developed for higher density recording as compared to longitudinal media. The thin-film perpendicular magnetic recording medium comprises a substrate and a magnetic layer having perpendicular magnetic anisotropy. Typically, the thin-film perpendicular magnetic recording medium comprises a rigid aluminum (Al) alloy substrate and successively sputtered layers. The sputtered layers can include one or more underlayers, one or more magnetic layers, and a protective overcoat. The protective overcoat protects the magnetic layer from corrosion and reduces frictional forces between the disc and a read/write head. In addition, a thin layer of lubricant may be applied to the surface of the protective overcoat to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
Granular perpendicular magnetic recording media is being developed for its capability of further extending the areal density as compared to conventional perpendicular recording which is limited by the existence of strong lateral exchange coupling between magnetic grains. A granular perpendicular recording medium comprises a granular perpendicular magnetic layer having crystalline cobalt-based magnetic columnar grains separated by grain boundaries comprising voids, oxides and/or nitrides. The grain boundaries having a thickness of about 2 Å to about 20 Å, provide a substantial reduction in the magnetic interaction between the magnetic grains. In contrast to conventional perpendicular media, wherein the perpendicular magnetic layer is typically sputtered at low pressures and high temperatures in the presence of an inert gas, such as argon (Ar), deposition of the granular perpendicular magnetic layer is conducted at relatively high pressures and low temperatures and utilizes a reactive sputtering technique wherein oxygen (O2) and/or nitrogen (N2) are introduced in a gas mixture of, for example, Ar and O2, Ar and N2, or Ar and O2 and N2. Alternatively, oxygen or nitrogen may be introduced by utilizing a sputter target comprising oxides and/or nitrides which is sputtered in the presence of an inert gas (e.g., Ar), or, optionally, may be sputtered in the presence of a sputtering gas comprising O2 and/or N2 with or without the presence of an inert gas. Not wishing to be bound by theory, the introduction of O2 and/or N2 provides oxides and/or nitrides that migrate into the grain boundaries, thereby providing a granular perpendicular structure having a reduced lateral exchange coupling between grains. However, the migration of oxides and/or nitrides under low atomic mobility deposition (i.e., low temperature deposition), as well as the shadowing effect of the high gas pressure reactive sputter process, produces a granular magnetic layer having a porous structure significantly more susceptible to corrosion. Furthermore, the signal-to-noise ratio (SNR) of the granular magnetic medium needs to be enhanced for high-density magnetic recording applications.
The continuing drive for increased recording areal density in the magnetic recording media industry mandates reduction of the head-to-medium separation, or more particularly the head to magnetic layer separation. As such, an increase in areal density usually requires a reduction in the thickness of the protective overcoat which constitutes part of the head to magnetic layer separation. One role of the protective overcoat is to prevent corrosion of the underlying magnetic layer, which is an electrochemical phenomenon dependent upon factors such as environmental conditions, e.g., humidity and temperature. A suitable protective overcoat must prevent migration of ions, such as cobalt (Co) and nickel (Ni), from underlying layers to the surface of the magnetic recording medium, thereby potentially forming defects such as asperities. However, as the protective overcoat thickness is reduced to below 40 Å, the magnetic layer becomes more vulnerable to corrosion. Such a low thickness reduces the ability of the protective overcoat to maintain adequate corrosion protection.
Accordingly, there exists a need for granular perpendicular magnetic recording media having a granular magnetic layer exhibiting enhanced magnetic properties and improved corrosion resistance. In particular, there is a need for granular perpendicular magnetic recording media suitable for high areal density and reduced head-to-medium separation applications, exhibiting enhanced magnetic properties and improved corrosion resistance while simultaneously providing improved contact start-stop performance.